Pulse code modulation system



Dec. 9, 1952 A. \EST1 2,621,323

PULSE coDE-MoDULAT1oN SYSTEM Filed Feb. 26, 194e 2 sHEETsfsHEET 1 w N D@ T T A VN l i w UGA ,uw u nu Patented Dec. 9, 1952 PULSE CODE MUDULATIGN SYSTEM Arnold Lesti, Brooklyn, N. Y., assigner, by mesne assignments, to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application February 2.6, 1948, Serial No. 11,262

10 Claims.

This invention relates to a system for converting pulse amplitude modulations into pulse code modulations and more particularly to a converter system utilizing a cathode ray tube.

Various methods and systems :for converting pulse amplitude modulations into pulse code modulations have been proposed in the past. These systems may be roughly divided into two groups, one employing an indirect conversion method where electronic sampling is resorted to and a second group where a direct conversion of the amplitude variable signal pulses takes place into equivalent pulse code modulations. In the latter group cathode ray tubes have been known as coders or converters for some time.

It is an object of this invention to provide a cathode ray tube type converter which is eiective in producing simultaneously all parts oi the pulse code corresponding to a given input level or amplitude.

A further object is to provide a cathode ray tube pulse code modulation converter wherein the difliculties due to alignment are greatly minimized.

In accordance with certain features of the invention I provide a pulse amplitude to pulse code modulation converter essentially comprised of a cathode ray tube which includes as the target for the cathode ray beam a given number of coding bars or other electron beam targets arranged in parallel or in other rows and which are swept by the beam of the line type simultaneously. 'Ihe coding bars include high and low secondary emission portions which are arranged on the various bars so as to provide for each given amplitude level as expressed by the deection of the beam, a predetermined code pulse combination. Any given number of channels may be combined by the use of suitable keying. In order to improve the operation of the system a beam centering circuit and a code quantizing circuit has been provided, as a means for controlling the operative stability of the system.

The above mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. l is a diagram partly in block form illustrating the circuit elements of a complete conversion system in accordance with the present invention;

Figure 2 is a plan view of the cathode ray beam target in the cathode ray converter tube 2 illustrating the coding bars of the system in Figure 1;

Figure 3 is a cross-sectional view of one of the bars shown in the plan of Figure 2;

Figure 4 is a representation in graph form oi sample amplitude modulated pulses which are to be converted by the system in accordance with the invention, and

Figure 5 is a graphical representation of an amplitude modulated pulse to illustrate the principle of quantizing in the system of Figure l.

Referring to the conversion circuit shown in Figure 1, a number of audio input channels indicated at I are connected to feed a common pulse amplitude modulator 2 which receives suitably timed unmodulated pulses from a pulse generator 3. The pulses of the various channels which form a consecutive train or pulse sequence and which are obtained from the modulator 2 modulated in their amplitude according to the instantaneous value of the signal in the respective channels, are fed into a translater and mixer circuit 4 and from there through a push-pull amplier 5 are applied in push-pull to the two horizontal deflection plates 6 and 1 of a cathode ray type pulse code modulation converter tube 8. The converter tube 8 is comprised of conventional operating elements including an electron gun 9, for producing a line type beam, a set of vertical and horizontal deflection plates I Il, a collector electrode II and a number of coding bars I2, acting as a target for the cathode ray beam in the tube 8. In addition a set of center of modulation stabilizing bars are provided as indicated at I3. As will appear in connection with the detailed showing of Figure 2, all of the bars I2 and I3 are swept simultaneously by the cathode ray beam. These bars provide an individual output to their respective translating circuits I4 as shown for example for the rst bar. Signals obtained from the circuits Iii are combined in a mixer circuit I5 by way of suitable individual gating circuits IS.

These gating circuits are controlled by means of pulses obtained from the pulse generator 3 and a suitable delay circuit 3A over a connection I'I. In order to obtain so-called quantizing of the line beam deflection with respect to the code pulse producing portions of the coding bars a control circuit is provided which derives a signal proportional to the output of one of the coding bars as shown for the purposes of illustration for the rst bar at a point I8 between the translating and gating circuits I4 and It respectively. The output of the coding bars is combined in a one to the other of its two stable conditions and;`

thus provide an additional voltage increment for the amplitude modulated pulse as applied tothe mixer circuit 13. The line beam .is given thereby additional deiiection to one side or the other which is suiiicient to place itastr-ide an indica-j tion producing spot on the coding bars.

A stability control circuit .is also provided .to maintain stability in the symmetry of defiection of the cathode ray beam with respectto the center point of operationl which would correspond c to the ypoint where .no audio modulation is present. This circuit is fed from'the two halves of-a stabilizing bar indicated at It. .The output of the stabilizing bar .portionsgis iediintofatwin ampliiier. 23V and Y from there into atwin A.or dual type rectiiier 25%. Thefrectiiieroutput .ofthe stabilizing. bars is then applied. througha .pushpull ampliiier 25 as an additionaldeiiector voltage to the Vtwo horizontal deflection plate'sfand 'I in accordance with the. resultant ldiiierential signals obtained from 'the two stabilizing Ibar portions, one' orthe other sidebeing adjusted .i0 Cellier the beam Proreflv- .fAfs'rOsS .manual control for beam centering -is pro vided as ,atf26.

-Thecoding and stabilizing barswill-nowbe discussed .in detail With. efeeiet'tofgulfe i2- 'It is seen therefrom thatatotal ef ve parallel .coding-barshave Abeen p rovided indicated at .2l through 3 I respectively. Although theI view shows r:five coding bars fora design.employinggathirtytwo code system, actuallya muchgreaterrnumber is vgenerally contemplated in .accordancewith the desired accuracy of reproduction for the P. v(pulseamplitude) to P.-C. M. (pulse 00de modulation) .CQnverSiQn- @The Code barsare insulatedfromeach other and alead vis cornnected'to each and broughtout ofnthe tubeffor connection to associated circuit components'the leads being-indicated at-*32. All of the coding bars are h eldltogether mechanically onan-:insulating form of glass or other suitable material. The code arrangement itself isy etched out ofV the bars preferably by a photoengraving process. The .portions etched out as shown lin the sectional .view in Figure 3 are lled. with a materialsuch as colloidal carbon-to provide low. secondary electron vemission in response `to theimpingement vof the cathode ray beam, the remaining portions being zsensitized fora lhigh secondary emission. The line beam isshowninrelativefsize at 33. It isseen therefrom that. asimultaneoussweep- Ving of all the target rows orbars may b eachieved, the deflection along the lengthof-the bars being proportional to the applied amplitude variable pulse. -The length dimensions of the bars` and the location of the .related code producing portions of the bars issuch that they correspond to a predetermined number of levels into which the maximum foreseeable amplitude of the variable amplitude pulse has been divided. Y1n the present instance .thirtytwo .such levels have been lchosen .to represent the total maximumamplitudef Thusas'la result of theapplication to the .two horizontaldelection .plates 6 and .7 .of la puisepfa gyenampinude mennebeam will.. Shift.

.4 rapidly in a horizontal direction and will come to rest in a position corresponding to the voltage of the particular amplitude pulse modulation channel supplying the deiiecting pulse, for a certain period of time. For any given amplitude only the beam will come to rest on certain bar portions which-will give a. comparatively high secondaryY emission in-accordance with preestablished coding. For instance, in the example shown in Figure 2 the bars l, 3 and are being activated. The high secondary emission portions Alof "the'barishown in cross section in Figure 3 ,are indicated at. 34 and the low secondary emis sion portion. which has been filled with colloidal carbon are sh'own at 35. In Figure 4 representative ampliilde'modulated pulses have been shown for thesixchannels illustrated in Figure l. In

Yltl'ii's'view thel'ineshown at 36 indicates the level V of z ero modulation corresponding to the midpoint ofthe two center of modulation stabilizing These i stabilizing bar. ,members have. Yuniforr-.ily

high rate Vof..*Sf.Ondary ernissionand .each bar is connected to va ..correspending Y,half of 4,a twin amplifying circuit-23 asalready. discussed in connection with Figure 1."

In operatingthefsystem discussed above, pulses ofthe type shown in Figures .4..and5iwill be obtained from. the` pulseT amplitudeA modulating circuit-2. Theinitialamplitude modulated pulse as eiective `with' respect. to. thedeiiection.I plates 6 and; 1, would be as indicatedat (Fig. 5). The line beamwill be made toQsweepacross-the five coding bars vand will ycome to.r est. .entirely in a position..corresponding .to the amplitude V39. lf thislposition fof the beam s lfioulcl` vbe productive of al signaLj thatfisif. thebeamshould. come to reStastride aset Vof highise'condary-emissionportions onfthe -bars,- rio-"furthercorrection will take place in respect to the deflection ofthebeam.

ASuch abea m. position would mean that-*the beam, `which has a .-suiicient width to be Y somewhat to a certain predetermirildvoltageflevel. VThese kcoding pulsesL are thusemade;.of;.si11cient .amplitude. to actuate. the'` translating circuit I which preferablyincludejs .afstorage tube circuit and yfrom-there .are fed intothe gating'circuit which alsol actsfas a. distributor :for the pulse signals. Aslalready .indicated .this Adistributor circuit is connected to thetiming lgenerator so as `to be synchronized or gated 'therefrom to a different time .period for .each of Ythecoding bars. /The result in the-commonoutput isa pulse code modulatedsignal correspondingto theamplitude of theapplied pulse vwhichl is vapplied ltoother circuitsfor further'y processing. *When scanning the coding'bars` it isapparentthatduelto the arrangement of thebars 4'and the line beam the code itself will be produced r-simultaneously and vwiil enable al`ignment`d ifculties to basu'bstannani' eliminated.' seguid the initial resection of the eiectron beamio'eiiample corresponding .i0 thgl'erelf einlegen@aSlSheWh in Fia 5 be 'Such .as ,t0 sans@ the lbam "initially fio. dem@ to rest between coding position indicating por- .tions yof .the laars. 1.19 .eode-ipdicaiion-noiild be `,sitterin?i.' lllhe .consequent ...lack .of output .from

the five bars will cause deblocking of the keying circuit 2| and produce a pulse into the ilip flop circuit 22. A pulse from the circuit 22 will thus be obtained which when added to or subtracted from the amplitude of the pulses responsible for the horizontal deflection of the beam, will cause the resultant operative amplitude or the original deflecting pulse to be increased or decreased to a value as at 49 or li depending on the relative position of the line beam. As a consequence the beam will be shifted to the left or the right to assure a code indication corresponding to the nearest corresponding one of the thirty-two `amplitude levels. In order to overcome the operative instability of the center point of the operation, that is, the code corresponding to no audio modulation, the second control. circuit discussed in connection with the stabilizing bars 3l and 38 has been provided. The pulses from each of the bar portions 3'! and 38 are amplied separately in the twin ampller 23 and are separately rectied in the circuit 24 which is effective in producing positive D. C. voltages applied to the grids of the push-pull amplier 25. If both stabilizing bars receive the same number 0i beam excursions the rectified outputs of the circuit 24 will be the same and voltage into the two grids of the push-pull ampliiier 25 will be the same. This voltage being in phase will not be amplied and hence will have no effect on the b-ias voltage of the deeotion plates. If, however, the stabilizing bars are exposed to different beam sweep portions, due to a shift in lthe center point at which the line beam would come to rest with or without modulation, the beam Will act on one bar portion with respect to the other for a comparatively longer period of time. In that case the rectified output of one side of the rectier 2Q would be different from that of the other side. Hence there would be a difference in voltages which is applied to the grids of the push-pull rectifier 25 which will be amplified and cause a change in the voltage applied to the deiiection plates. This is in such a direction as to bring the beam back onto the center.

If a tube producing a line beam is not being used in accordance with the above discussion the vertical deflection plates shown at 42 and 43 may be used for a vertically directed scanning signal having a saw-tooth or sinusoidal characteristic. The frequency of such a sweep will then be synchronized with those of the time period o1' duration of the applied amplitude modulated pulses. As an alternative the frequency may be much higher to produce in eiect the equivalent of a line beam.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

What is claimed:

-1. An electronic system for converting an amplitude modulated signal into a given multiple element pulse code indication comp-rising an electron dis-charge device having a plurality of target strips arranged in accordance with said code, means for projecting an electron beam on to said target strips of suicient width to simultaneously touch all said strips, and means responsive to the amplitude of said signal to deiiect the beam along the strips to a position corresponding` to the code representation of said amplitude.

2. An electronic system for converting an amplitude modulated signal into a given multiple element pulse code indication comprising an electron discharge device having a plurality of rows of target elements arranged in accordance With said code, means for projecting an electron beam on to said target elements of suflcient width to simultaneously touch all the rows, and means responsive to the amplitude of said signal to deect the beam along the rows to a position corresponding to the code representation of said amplitude.

3. An electronic system for converting an amplitude modulated signal into a given multiple element code pulse indication, comprising means for supplying pulses each modulated from a given amplitude in accordance with the instantaneous amplitude of a variable amplitude audio signal at a dilerent one of succesive intervals of time; means for simultaneously producing a multiple element code pulse indication corresponding to and directly in repsonse to each pulse, said producing means comprising a cathode ray tube pulse amplitude to code pulse modulation converter having an electron gun, a set of electron beam deflection means and a target having a number of parallel bars disposed for simultaneous sweep by the cathode ray beam; and means forming an output circuit for said system operatively connected to said producing means.

4. An electronic system according to claim 3 wherein said target includes center of modulation stabilizing bar portions.

5. An electronic system according to claim 3 further including means for maintaining stable the -center of modulation of said amplitude modulated signal With respect to said code pulse producing means.

6. An electronic system according to claim 5 wherein said maintaining means includes secondary electron emitting bar portions in a cathode ray tube and a circuit for providing electron beam deiiection bia-s controlled from said bar portions.

7. An electronic system according to claim 3, further including means for quantizing the amplitude modulated pulses.

8. An electronic system according to claim 7, wherein said quantizing means includes la plurality of target bars in a cathode ray tube having high and low secondary electron emitting portions and a circuit responsive to absence of code pulse signals in said output circuit means for providing a cathode ray beam deillec-ting voltage.

9. For use with a pulse amplitude to binary pulse code converting system, a pulse converter comprising a cathode ray tube including an electron gun for producing a line type cathode ray beam, a set of deflection plates for said beam, and a code pulse producing target for said beam comprising a plurality of parallel bars having loW and high secondary electron emission portions disposed to provide a plurality of simultaneous code pulses when exposed to said line beam and a pair of secondary electron emitting bar members for stabilizing the center of modulation of said amplitude modulated pulses.

10. An electronic system for directly converting an amplitude modulated pulse signal into binary code pulse indications, comprising a multichannel amplitude to pulse amplitude converter; a cathode ray tube for directly converting said pulse amplitude modulated signals into binary pulse code signals including electron gun and beam deflection means, for producing a line type beam, a target for said beam having a plurality of parallel bars having high and low secondary v7 selectron emitting .portions .for .simultaneous .UNITED STATES PATENTS lSweep by saidl beam; :anoutput circuit. for each Number Name Date .of esaid coding bars including .Circuit gating 171150 Shelly Aug v29 1939 lmeanscircuit means 'forcombrliug the Output .2,417,450 Sears u Mar` 18 1947 of -sadoutput circuits; quantizing circuit means 5 .2429631 Labn 93 u Oei; 23 1947 controlled fromsaid output circuit including a .2,445,568 Ferguson n Juli; 20I 1948 flip-.flop circuit-and means connecting said ip- 27458652 `SEMS n Jan 11 1949 op circuit with said beam defeCtug means; 453,535 Hecht Mar 8J 1949 means for stabilizing `the centerof modulation of 2-473691 ,11/1emh3r;l 511116.21 1949 .said-amplitude modulated pulses with respect to 10 n .said `target includingsecondary electron emitting OTHER REFERENCES bar portions forming part .of said target, acontrolcircuit including atWin amplier connected to.saidrbar.portions-and a dual rectersupvplied from said twin amplier, and means con- 15 Ynecting saiddual .rectifier and said defiecting means for .centering said beam, and synchronizingpulse generating-means connected to control Puls? Code Modula'fjlqn' by Sears pages 44-57 0f Sadconverr and Sadrgang means BellSystem VTechnical,Journal, vol. XXVII, No.l

y .Lann .19.48. ARNOLD LEsTI. 20 a ary --A1:tiele,"An Experimental Multichannel Pulse CodeModulationSystem, etc. by Meacham et al.,` pages 1-43 of Bell System Technical Journal, vol. XXVII, No. 1, January 1948.

VArticle,"Electron Beam Deection Tube for l REFERENCES .CITED :ThefolloWiI-lg references are .of'recoid inthe -fue of this patent: 

